In recent years, an ink jet recording method has found wide application in various kinds of graphic art fields such as photography, various kinds of printing, marking and specific printing such as a color filter, since it can form images easily and inexpensively. Particularly, it has also become possible to obtain image quality comparable to silver salt photograph images by utilizing a recording apparatus, which ejects and controls fine dots; ink in which a color reproduction range, durability and ejection suitability have been improved; and exclusive paper in which ink absorption, color forming property of colorants and surface gloss have been greatly improved. Image quality improvement of an ink jet recording method of today has been achieved only when a complete set of a recording apparatus, ink and exclusive paper is prepared. Image quality improvement of an ink jet recording method of today has been achieved only when a complete set of a recording apparatus, ink and exclusive paper is prepared.
However, an ink jet system which requires exclusive paper is problematic in respect to limitation of a recording medium and cost up of a recording medium. Therefore, many attempts have been made which record on a recording medium different from exclusive paper, employing an ink jet recording. Concretely, there are methods such as a phase-conversion ink jet method utilizing wax which is solid at room temperature, a solvent-type ink jet method utilizing an ink which is mainly comprised of a rapid-drying organic solvent and a UV ink jet method in which an ink is cross-linked by ultraviolet (UV) light after recording.
Among them, a UV ink jet method has been noted recently in respect to odor relatively lower than that of a solvent-type ink jet method, rapid drying property and capability of recording on a recording medium having no ink absorption. Various UV-curable ink jet inks are disclosed (for example, in Japanese Patent Publication Nos. 6-200204 and 2000-504778).
It is known that the UV ink can be divided mainly into a radical polymerization type and a cationic polymerization type. In an ultraviolet curable ink jet recording method, image quality, i.e. the diameter of dots, is controlled by factors such as light exposure timing after ink has been ejected, intensity or energy of exposed light, ink droplet size, photosensitivity of ink, surface energy, ink viscosity, wettability of recording medium, pattern of dots, and error diffusion pattern. Specifically, factors, which mainly affect image quality, are photosensitivity, viscosity, and surface tension of ink, wettability of recording medium, and exposure conditions. In the radical polymerization type, photosensitivity largely depends on ink layer thickness and exposure illuminance, since polymerization is inhibited by oxygen, while in the cationic polymerization type, photosensitivity largely depends on humidity as well as temperature.
In order to minimize the polymerization inhibition due to oxygen In the radical polymerization type, known are a method which employs monomers, initiators and initiation aids which are not influenced by oxygen, and a method in which oxygen purging is performed employing inert gases such as nitrogen.
In order to minimize the humidity dependency in the cation polymerization UV curable ink, known is a method in which ink after ejected is heated (see for example, Japanese Patent O.P.I. Publication No. 2000-137375). Known as cation polymerizable monomers employed in the cation polymerization UV curable ink are an epoxy compound having an oxirane ring, an oxetane compound, and a vinyl ether compound.
In particular, it is known that a combined use of an epoxy compound and an oxetane compound markedly increase polymerization rate. For example, pertinent publications include Toa Gosei Kenkyu Nenpo (Toa Gosei Annual Research Report) TREND No. 2 (1999), “Oxetane Kagobutsu no Hikari Cation Koka System eno Oyo (Application of Oxetane Compounds to Cation Photocurable System of Oxetane Compounds)” and Japanese Patent O.P.I. Publication No. 2679586”. Specifically, since the oxetane compound results in excellent heat resistance, adhesive property, and chemical resistance, it is useful to use the oxetane compound together with the epoxy compound which enhances reactivity.
Japanese Patent O.P.I. Publication No. 2001-220526 discloses application of this technique to ultraviolet curable ink jet printing. In recent years, the ultraviolet curable ink jet recording system, in which ink is cured by ultraviolet radiation, has been noted as an image formation method employing a recording medium with less ink absorbability.
Generally, well known and practically used as ultraviolet curable ink is a radical polymerization type ink. On the other hand, cation polymerization type ink exhibits advantages such that polymerization inhibition due to oxygen, as found in the radical polymerization type ink, does not occur, low intensity light sources are usable, unpleasant odors usually generated by acryl monomers are not produced, and components used are less irritant, however, it has not been put into practical use.
The reasons is that the radical polymerization type ink has properties in that photosensitivity markedly decreases under high humidity and depends on temperature. Such an ambience-dependent ink has substantial problems in which image quality varies depending on various ambience.
Japanese Patent O.P.I. Publication No. 2002-137375 discloses a method in which cation polymerization type ink is employed, and ink to have been ejected is heated and subsequently exposed to light. However, from the viewpoint of printer cost and use of low heat resistant components, the use of the heating mechanism does not seem to be preferable. Further, Japanese Patent O.P.I. Publication No. 2001-181386 discloses 2-(4-metoxyphenyl)-3,3-dimethyloxetane as a compound to increase the reaction rate. In an ink composition containing an oxetane compound such as di[1-ethyl(3-oxetanyl)]methyl ether and an alicyclic epoxy compound, the alicyclic epoxy compound can be replaced with the above reaction rate increasing compound. Reactivity of the ink composition in which the alicyclic epoxy compound has been replaced with the above reaction rate increasing compound is equal to that of the ink composition before replaced. However, when low intensity light sources such as a fluorescent lamp are employed, the ink photosensitivity is not sufficient under high humidity ambience.